Substituent Effects and the Mechanism of Gold to Alkene Benzylidene Transfer Employing a Gold Sulfonium Benzylide Complex

Aliphatic and aromatic alkenes react rapidly and efficiently with the gold sulfonium benzylide complex [(P)­AuCHPh­(SPh2)]+ {B [3,5-CF3C6H3]4}− [P = P (t-Bu)2o-biphenyl; 1] at room temperature via the cationic two-coordinate gold benzylidene complex [(P)­AuCHPh]+ (I) to form phenyclyclopropanes. The reactivity of p-substituted vinyl arenes toward I decreased with the decreasing electron donor ability of the vinyl arene (ρ = −1.49 ± 0.15), and the reactivity of aliphatic alkenes toward I depended strongly on the number of alkyl groups attached to the more substituted alkene terminus. Benzylidene transfer to cis- and trans-β-methylstyrene occurred with complete retention of the alkene configuration. Benzylidene transfer to vinyl arenes led to predominant formation of cis-cyclopropanes (dr = 4.1: 1 to 13.9:1), whereas benzylidene transfer to aliphatic alkenes was generally less selective (dr = 1.3:1 to 5.1:1). All our experimental observations were consistent with a concerted, asynchronous, electrophilic benzylidene transfer mechanism. The diastereoselectivity of benzylidene transfer was interpreted in the framework of a transition state model involving a near anti-periplanar arrangement of the carbene AuC and alkene CC bonds.